Tuned circuits



Aug. '17, 193?. P. o. FARNHAM 2,090,513

TUNED CIRCUITS Filed June 13, 1930 4 Sheets-Sheet 1 P. o. IFARNHAM 2,090,513

"TUNED CIRCUITS Aug. 17, 1937.

Filed June 15, 1930 4 Sheets-Sheet 3 I000 woo M00 fff/ocyc/es Patented Aug. 17, 1937 TUNED CIRCUITS Paul 0. Farnham, Boonton, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation Delaware Application June 13, 1930, Serial No. 461,020

3 Claims. (01. 178-44) This invention relatesto tuned circuits and particularly to tuned 'circuitszgfor use in-ra'dio frequency or carrier wave transmission systems. i.

' An object of the inventionisto provide coupled 5 circuits that exhibit selectivity and transmission characteristics throughout the entire range of frequencies over which they may be tuned. A

:-further object is to provide asystem of tuned wcircuits having fixed coupling units between the circuits; the units being so chosen that the coupling between the tuned circuits may be main- .tained approximately constant throughout the tuning range. More specifically, an object is to provide a tuning system including two capacitively coupled circuits which may be continuously tuned to resonate at the same frequency, the coupling between the tuned circuits including series and shunt capacities of such values that the variation, with frequency, of the total coupling between the tuned circuits is substantially reduced. These and other objects of the invention will be apparent from the following specification when taken with the accompanying drawings, in which, ,Fig. 1 is a circuit diagram showing one embodiment of the invention employed as a coupling between a collector system and a carrier wave amplifier,

Fig. 2 isa circuit diagram illustrating the invention as embodied in an interstage coupling,

Fig. 3 is a curve sheet showing the variation,

system A, G, and the first tube T of a carrier wave amplifier.

The tuned input system comprises substantially identical L C circuits in which the high potential terminals of the adjustable condenser C and fixed inductanceL of the separate tuned circuits'are connected, respectively, to the antenna A through a coupling condenser Co, and to the control grid G1 of the tube T. The low potential terminals of the condensers C are connected to each other and to ground, and the low potential terminals of the inductances L are joined and are connected to ground through the 56 coupling condenser Cm. For feeding a-direct current bias to the control grid, the condenser Cm may be shunted by a fixed resistor R1.

In addition to the coupling condenser Cm, the tuned circuits are coupled by a second capacity Cm between the high potential ends of the tuned circuits. For convenience of description the cou pling capacities Cm and Cm will be referred to as shunt and series couplings, respectively, since they bear these relations to the general direction of transmission along the collector-amplifier systerm with a common adjusting means, such as indicated diagrammatically by dial D and shaft S, to secure simultaneous adustment of the circuits to resonance at the same frequency.

As shown in Fig. 2, substantially the same arrangement as above described may be employed as an interstage coupling between successive tubes v T, T, of a multistage amplifier. For such use,

The two LC circuits are -preferably provided the inductance L of the first tuned circuit preferably forms the secondary of a transformer which has as its primary winding the plate inductance L1 of the tube T.

For simplicity, the circuit diagrams constituting Figs. 1 and 2 do not illustrate the sources of plate current supply and the cathode-heating circuits, but it will be understood that the usual or any desired systems maybe employed for energizing the amplifier tubes.

, The coupling between the tuned circuits includes both series and shunt capacitive couplings but magnetic coupling is substantially zero. At the higher frequencies the effects of the series coupling Cm predominate and at the lower frequencies the effects of the shunt coupling Cm are the more pronounced. The values of the coupling capacities are preferably so chosen as to give less than critical coupling between the two tuned cir' cuits throughout their entire tuning range, 1. e.. at any adjustment of the tuning condensers the transmission along the system is a maximum for but one frequency and not for two slightly separated frequencies: To accomplish this result, the maximum value of the coupling due to shunt capacity Cm must be somewhat below the amount necessary to produce critical coupling at the low frequency end of the tuning range, and the maximum value of the coupling due to the series capacity Cm must be somewhat less than the amount necessary to produce critical coupling at the high frequency end of the tuning range.

If the coefllcients of coupling between the circuits due to Cm and Cm are represented by k and 2 I k, respectively, their values are given very closely by the equations:

where. C is the tuning capacity in each circuit. Then K thetotal coefiicient of coupling between the circuits is:

and, since The frequency variation of the several coupling coefilcients, k, k and K is shown graphically in Fig. 3 by the curves identified by these reference characters. The curve P of this curve sheet shows the relation between frequency and the power ratio,

of coil L of one of the tuned circuits. In this particular case, the coils L were identical and comprised single layer solenoid such as commonly employed in broadcast receivers.

It will be noted that the graph K for the co- .efiicient of total coupling has approximately the the coupling may be more or less than critical vent cutting oiv the side bands", when same shape as the graph P for the power ratio of either inductance. In the more general case where the power ratios of the two coupled circuits are not substantially equal, the quantity,

P, to which thetotal coefilcient of coupling, K,

is to .be referred is given by the geometric mean of the power ratios of the separate tuned circuits. i. e., by'the square root of the product of the individual powerratios. Whether in the general case, or in the special case in which the tuned circuits are substantially identical, the coupling between the circuits will be less than critical for all frequencies at whichthe coupling coemcient .K is less than the quantity P. For the particular circuits or coupling system from which the data for Fig. 3 was obtained, the coupling was less than critical throughout the entire tuning range since curve K does not rise above curve at any point.

The invention is not restricted to circuit arrangements in which the couplingis less than' critical but provides a general method of coupling circuits in such a way as to give a desired gain versus frequency characteristic. If desired,

at one part of the frequency range and may approach or pass beyond the critical value at another portion or portions, of the range. For

some purposes, such as, for example, in cascaded 4, 5 and 6 which show the relation between frequency and voltage step-up in certain particular embodiments of the invention. In both the input and the interstage coupling systems, the several inductances L had the same value of approximately 220 microhenries and took the form of 100 turns of No. 30, enameledwire closely wound on a 1% inch tube. The adjustable condensers C were units of a gang condenser and had such value that the system could be tuned over the broadcast range of from 500 to 1500 ,kilocycles. In the interstage coupling, the plate inductance L1 consisted of 50 turns of No. 36 double silk covered wire wound, turn for turn, over inductance L beginning at the low potential end thereof.

In Fig. 4, the curves A, B and C show the relationship of voltage amplification and frequency for different values of the antenna coupling,

which in thecircuit of Fig. l was a capacitive coupling-provided by the condenser Co, the shunt coupling capacity being .02 microfarad and the series capacity being 0.4 micromicrofarad. The

antenna or collector system. Itis to be noted that for each 'value of the condenser Co, the voltage step-up remained approximately constant for frequencies above about 900 kilocycles. Since the usual antenna input circuits exhibit decidedly higher gains at the higher frequency, it is evident that the invention provides a system which is particularly useful when uniform gain at the high frequency end of a wave band is desired. The same control of the gain-frequency characteristic obtains if the capacitive coupling to the antenna is replaced by a magnetic coupling.

The effect obtained with dillerent values of the series coupling Cm will be apparent from a consideration of the curves of Fig. 5. The solid line curve 1) represents the operation of, the Fig. 2 circuit when the shunt capacity Cm was .03 microfarad and the series coupling between the tuned circuits was omitted. i. e., Cu is equal to zero. The drop in amplification E/Eo at the higher frequencies is very pronounced. When series coupling is introduced, the amplification curve remains substantially unchanged at the lower frequencies, 1. e., up to about 1100 kilo-' cycles for the circuit elements as stated above,

but dotted curves E, I" and 6 show the increased gain at higher frequencies when the series condenser Cm' had values of 02,04 and 0.6 micromicrofarad, respectively. I

The curves of Fig. 6 show the relation between amplification and frequency-for different values of the shunt capacity Cu in thecircuit of Fig. 2. Curve! is identical with curve F of Fig. 4 and represents the operation of the coupled tuned circuits for a series capacity of 0.4 micromicrofarad and shunt capacity of .03 microfarad.' Curves H, I, L are similar curves as obtained when the capacity had values of 02, .04 and, .05 microfarad,

The curves of '7 afford a comps'risonbetweengthe selectivity of one particular ment of the invention and the selectivity of the conventional system employing two cascaded stages having tuned output circuits. The data for the curves was obtained bytuning theamplifiersto-resonanceateachofthreedif- 75 resonant frequency and also at frequencies di'fierf will reproduce the modulation frequencies. at

.' their proper values.

in the side bands of a modulated signal are not ing from that to which the amplifier was tuned. The solid line curves' 600, 1000 and 1333, show the selectivity, for resonant frequencies of 600, 1000 and l333 kilocycles, respectively, of the amplifier circuit of Fig. 2 when the series and shunt capacities had-values of 0.4 micromicrofarad and .03 microfarad, respectively.

The dotted line curves 600, 1000 and 1333 are similar selectivity curves for the conventional amplifier arrangement employing but one tuned circuit in each of two cascaded stages. These dotted line curves are typical of the known tuned amplifiers and show a marked decrease in selectivity at the higher frequencies. The invention provides a', more uniform selectivity'throughout the frequency band over which a transmission system may be operated.

It is particularly to be noted that the selectivity curves representing the operation of one embodiment of the invention substantially coincide for a zone of approximately three kilocycles off of resonance. The significance of this feature is that it is possible to design a receiver system which, without the use of band pass tuners All of the frequencies lying amplified at the same rate, in the radio frequency ammifier, but the audio amplifier may be designed to correct for this differential amplification. Such a corrective audio amplifier system could not be employed with the known types of radio frequency amplifiers since, as shown by the dotted line selectivity curves, the suppression of the higher audio frequencies is not constant for all signal frequencies.

As shown by the several curves of Figs. 5 and 6, the particular values stated above were chosen to secure an approximately uniform gain at the higher frequencies. It is possible to secure even higher gains at the higher frequencies by increasing the value of theseries coupling. For radio.

reception and for other uses where high selectivity over a wide band of frequencies is desirable, the choice of the series coupling capacity is dependent, in part at,least, upon the maximum step-up which is consistent with high selectivity.

The invention is not limited, however, to a particular range of relative values of the two couplings but affords a general method for the design of tuned circuits and/or tuned amplifiers which are to exhibit predetermined transmissionfrequencycharacteristics. D

It will be understood that the coupling ca by a separate, physical condenser, or by stray coupling between the high potential ends of physical elements of the two tuned circuits, such stray coupling beingproduced in a manner so well known to those skilled in the art as to render unnecessary any further explanation.

While'the inventionhas been described in con- 1 .nection with certain embodiments that are pari ticularly adapted for the reception of broadcast will be apparent that the invention is equally signals Within the 500-1500 kilocycle band, it

There is considerable choice as tothe relative values of the series and shunt couplings and, within those ranges which give improved selecapplicable to' other frequency bands. Furthermore, at other frequency bands or when other gain-frequency characteristics are desired, it becomes-practical to employ inductive couplings in place of the capacitive couplings, or to employ combination inductive-capacitive couplings for either or both the series and shunt coupling elements.

I claim:

1. A tuned input crcuit for a radio frequency amplifier, comprising an "inductance having one terminal connected to the control grid of the amplifier, a coupling condenser connected between the, other terminal of said inductance and the cathode, a tuning condenser connected between control grid and cathode, a second inductance substantially identical with and having substantially zeroelectromagnetic coupling to said first inductance, a coupling capacity betweenthe grid terminal of said first inductance and one terminal of said second inductance, the other terminals of said inductances being directly connected together as regards direct-current,-a second tuning condenser connected across the non.-

commonterminals of said coupling condenser and said second inductance, a resistance connected in shunt across the coupling condenser, and means for impressing a signal upon the circuit comprising said second inductance and said second tuning condenser.

2. In combination with a source of signal energyand an amplifier circuit, a tunable band pass network comprising a pair of tunable circuits having a common condenser included therein, means for tuning said circuits to a desired frequency of a signal frequency range, the mag netic coupling between the circuits being subsary/ to produce critical coupling at thehigh frequency end of the tuning range.

3; The combination with two inductances, serially connected as regards directcurrent, having substantially zero electromagnetic coupling, of a capacitive coupling between thenon-common ends, of said inductances, a condenser having v one terminal connected to the commonjunction of said inductances,' a pair of tuning condensers having a common terminal connected to the other terminal of said first condenser, the opposite terminals of said tuning condensers being connected to the respective non-common terminals of said inductances a resistance connected in shunt across said first condenser, and mechanical pacity Figs. 1 and 2 may provided either 1 means connecting the adjustable elements of said ment of said tuning condensers, less than critical coupling.

PAUL o.

' seriescondenser is less than the amount neces- 

